Capacitive touch sensor devices have been incorporated into a range of electronic devices, including mobile communication devices, such as cellular telephones, computers, portable entertainment devices, appliances and touch screens, to name but a few. At a minimum, a capacitive touch sensor device includes one or more touch sensors, each of which is configured to indicate a capacitance change when, for example, the sensor is touched (e.g. by a stylus or a user's finger). Such changes in capacitance may then be processed in order to detect when the sensor has been touched.
In a conventional touch sensing application, a capacitance of the sensor is sampled and a baseline value is subtracted from the sampled value to derive a delta value. The delta value is then compared to a threshold value. If the delta value is greater than the threshold value, then a touch is detected, where as if the delta value is less than the threshold value then no touch is detected.
In order to accommodate for changes in ambient operating conditions and sensitivity changes, it is known to dynamically update the baseline value. For example, a typically technique for such dynamic updating of the baseline value is to implement a simple convergence of the baseline value to the sampled signal value when no touch is detected. In this manner, the baseline value will ‘track’ the sampled data value as long as no touch is detected.
However, touch sensing applications are often subjected to large amounts of electromagnetic noise effects, which can cause significant disturbances to the sampled capacitance signal. Specifically, such disturbances can cause fluctuations in the sampled data signal. Such noise fluctuations can result in the delta value for the sampled data dipping below the threshold value during a touch event, giving a false ‘not touch detected’ reading. Such false ‘not touch detected’ readings result in inappropriate tracking of the baseline value to the sampled data value during a touch event. As a consequence, the baseline value is inappropriately increased, increasing the vulnerability of the touch sensing components to noise fluctuations, and reducing the ability of the touch sensing components to detect touches.
In order to minimise the effect of electromagnetic noise, filtering may be applied to the sampled data prior to deriving the delta value. However, such filtering can reduce the sensitivity of the touch sensing components, making setting of the threshold value difficult for reliable touch recognition.